The present invention relates to an image-retransfer sheet to which an image such as letters, symbols, figures, etc. is thermally printed or transferred and from which the image is retransferred onto a surface of an image-receiving material with pressure. More specifically, the present invention relates to an image-retransfer sheet which is a base sheet for an image-transferring material.
Hitherto, instant lettering has been performed with using the image-transferring material, as follows:
An image such as letters, figures, symbols, etc. is thermally printed or transferred onto a surface of an image-retransfer sheet with using a heat sensitive image transferring device such as a printer, a typewriter, a word processor or the like, so that an image-transferring material having printed thereon the image is produced. In other words, the image is printed or transferred onto the image-retransfer sheet through a heat-sensitive image transfer process, so that the image-transferring material having printed thereon the image is produced. Thus obtained image-transferring material is then placed on a desired image-receiving material in such a manner that the image-bearing surface of the image-transferring material is brought into contact with a surface of the image-receiving material. In this state, the image-transferring material is pressed toward the image-receiving material under dry condition, as a result of which the image on the image-transferring material is transferred onto the surface of the image-receiving material. In other words, the image on the image-transferring material is retransferred onto the image-receiving material through a pressure-sensitive image retransfer process. Such an image-transferring material as capable of attaining the pressure-sensitive image retransfer function under dry condition will be referred to as "dry-processing type image-transferring material", hereinafter.
Japanese Unexamined Patent Application Publication No. sho-63-128987 discloses such an image-retransfer sheet onto which an image is to be transferred through the heat-sensitive transfer process to produce the dry-processing type image-transferring material. The image-retransfer sheet consists of a substrate film (such as a film of polyethylene, polypropylene, fluorine-containing resin, etc.) which has a smooth surface and exhibits a water-contact angle of at least 95.degree. or consists of a substrate sheet (such as a paper, metal foil, plastic film, etc.) coated with a silicone resin layer for treating the surface of the substrate sheet so that the substrate sheet may have a smooth surface and exhibit a water-contact angle of at least 95.degree..
In order to form or transfer an ink image onto such an image-retransfer sheet as having a water-contact angle of 95.degree. or more (particularly 105.degree. or more) through the heat-sensitive image transfer process, it is necessary to reduce surface tension of ink to wet the surface of the image-retransfer sheet and further necessary to increase adhesion of the image-retransfer sheet to the ink more than both cohesive force of the ink and adhesion between the ink and a polyethylene terephthalate (PET) sheet. For the purpose, the ink temperature has to be increased when the image is thermally printed, requiring high energy to be applied to the heat-sensitive image-transferring device, which is disadvantageous in view of durability of a thermal head and load on a power supply.
Since the conventional image-retransfer sheet has poor wettability, an image thermally printed on the image-retransfer sheet is liable to be retransferred with slight pressure applied thereto, due to weak adhesion of the image to the image-retransfer sheet. Accordingly, a portion of the image which is undesired to be retransferred from the image-retransfer sheet may unwillingly be retransferred, causing stains on the image-receiving material. Such easy retransfer is troublesome in handling of the sheet. In other words, the conventional image-retransfer sheet has a low rubbing-resistant property.
Furthermore, since the conventional image-retransfer sheet has an extremely small static friction coefficient, the sheet is liable to be moved with respect to the image-receiving material during the pressure-sensitive image retransfer process, so that the image is retransferred onto an undesired portion of the image-receiving material and the image retransferred onto the image-receiving material is distorted.
Japanese Unexamined Patent Application Publication No. sho-63-246298 discloses an image-retransfer sheet formed with a sticky layer at a position apart from the printed image for preventing the sheet from moving during the pressure-sensitive image retransfer operation. However, the formation of a sticky layer on a certain portion of the image-retransfer sheet necessitates specific means in production and an exclusive device therefor, requiring large costs. Furthermore, the sticky layer has to be covered with a separable sheet before use, i.e., before the image-retransfer step, which requires additional means and costs. In the case where the image-retransfer sheet has a repelable-type coating layer of silicone resin, in order to produce the image-retransfer sheet, it is necessary to coat a substrate of the image-retransfer sheet twice with the repelable coating layer and the sticky layer. In this case, regardless of coating order of the two, the later coating operation may be affected by the previous coating operation. That is, when a silicone resin is first coated on the substrate, the subsequent coating of a sticky composition is liable to be repeled. When the sticky composition is first coated, on the other hand, it is difficult to coat the silicone resin on the sticky layer because of stickiness of the previously coated sticky layer. In this case, if the sticky layer is covered with a separable sheet, the thickness of the resulting sheet partially increases so that the silicone resin cannot easily be coated. Contrary to the above description, even if the sticky layer is properly coated on the sheet, since the sticky layer is provided only at a certain portion of the sheet, the image-retransfer sheet is not fixed with respect to the image-receiving material at all the portions surrounding the image, and therefore the sheet is still liable to move during the image-retransfer step, resulting in formation of imperfect images on the image-receiving material.
In any case, a conventionally employed surface treating agent layer such as the silicone resin layer coated on the substrate of the image-retransfer sheet serves to ensure improved releasability of the ink image from the substrate. Therefore, the surface treating agent has been selected or formulated to have the property of reducing wettability of the substrate and decreasing adhesion of the ink image thereto, which property, however, deteriorates the heat-sensitive image-transferring property but necessitates a high thermal energy for the heat-sensitive transfer and deteriorates the image-rubbing resistance of the image-retransfer sheet.
In order to eliminate the above-described defects, Japanese Unexamined Patent Application Publications Nos.hei-2-81684 and hei-2-88294 have proposed that the surface treating agent provided on the image-retransfer sheet be retransferred toward the image-receiving material together with the image. The surface treating agent employed in the publications has a tensile strength in a range of 1 to 100 Kg/cm.sup.2. The surface treating agent has a melting point or softening point equal to or higher than 100 .degree. C. or has a melt viscosity at 100.degree. C. of equal to or higher than 1000 poises.
When the image-retransfer sheet coated with the above-described surface treating agent layer is subjected to the pressure-sensitive image retransfer process, however, not only a part of the surface treating agent layer provided below a position where the ink image is printed but also another part of the surface treating agent layer surrounding the ink image is liable to be retransferred onto the image-receiving material together with the ink image. Accordingly, in the case where the surface treating agent contains a colorant, there may occur a problem that a thickness of a line image retransferred onto the image-receiving material becomes extremely large or that an image retransferred on the image-receiving material is completely damaged to be hardly recognized.